Implantable medical devices, such as orthopedic and dental prostheses, can be made more permanent if the interface between the existing bone and the device contains some natural bone growth to knit the two components together. Such ingrowth has advantages over the use of bone cement, both in terms of stability and permanency.
“Bioactive” coatings on implantable medical devices allow for the ingrowth of natural bone into and around the device, forming chemical bonds between the device and natural bone. Bone is composed of substituted apatite crystals in an abundant collagen network. Type I collagen is the major protein of bone tissue, making up about thirty percent of the weight of bone. It has been shown that apatite crystals can grow and bond to collagen fibrils, and prepared apatite/collagen composites have been shown to promote direct bone apposition.
In addition to coatings, other materials made from apatite are used for bone repair and replacement. The cross-linked apatite/collagen porous scaffold materials have been studied for their excellent compatibility with human bone. Several approaches to preparing an apatite/collagen composite scaffold have been studied, but have exhibited drawbacks with respect to variable porosity of the composite.
One known approach is to prepare a composite material containing protein osteoinductive factor, mineral (mixture of hydroxyapatite and tricalcium phosphate) and collagen in a water suspension by a mechanical mixing means.
Another approach is by mixing insoluble collagen with calcium chloride and tribasic sodium phosphate at pH around 11.0. However, insoluble collagen was used to directly mix with apatite to form into composites, which may render an inhomogeneous apatite/collagen composite.
Finally, it is known to prepare an apatite/collagen composite using soluble collagen, phosphoric acid and calcium salt. Instead of forming apatite/collagen composite in one step, once the soluble collagen, phosphoric acid and calcium salt mix, the slurry-like mixture is freeze-dried. The gelation of collagen is carried out after freeze-drying apatite/collagen composite at a pH around 11.0. After gelation of collagen, the apatite/collagen composite is freeze-dried again to synthesis the apatite/collagen scaffold. Then the apatite/collagen scaffold is cross-linked and cleaned. This process requires two freeze-drying procedures and two cleaning procedures to form apatite/collagen composite scaffolds.
None of the above processes addresses the variable porosity of apatite/collagen scaffold, a factor that is important to control the regeneration of new bone tissue.
There remains a need in the art for improved apatite composite scaffolds, as well as improved processes to prepare porosity controllable apatite composite scaffolds.